1. Field of the Invention
The present invention relates to a vibration isolation device, an arithmetic apparatus, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
Active vibration isolation devices (to be simply referred to as vibration isolation device hereafter) are classified into an absolute vibration damping type and a relative vibration damping type. A vibration isolation device of the former type dampens vibration of a structure with respect to an absolutely still point using, for example, a skyhook damper scheme or a skyhook spring scheme. A vibration isolation device of the latter type dampens vibration of an object vibrating by itself, such as a structure which supports the device, and transmits vibration between two structures, that is, a structure supported by the device and a structure supporting the device.
To improve the vibration isolation performance of a vibration isolation device, it is a common practice to decrease the supporting rigidity of the device itself.
A control system of an active vibration isolation device (see, for example, Japanese Patent Laid-Open Nos. 2002-89619 and 7-93036) often includes a damping control system and a position/orientation control system. In recent years, a vibration isolation device of an absolute vibration damping type often uses an arrangement in which a floor vibration feed-forward control system (see, for example, Japanese Patent Laid-Open No. 9-184536) insulates vibration transmission from a support structure of the device to a support object of the device. Floor vibration feed-forward control is a method of mounting an absolute vibration sensor, such as a speedometer or an accelerometer, in a structure which supports a vibration isolation device, and feed-forwards its detection signal to an actuator of the device via a control unit.
An active vibration isolation device of the above-described absolute vibration damping type controls its support object to be still with respect to a virtual absolute still point. When seismic vibration occurs, a structure which is supporting the vibration isolation device and shaking upon the seismic vibration may come into contact with a structure which is supported by vibration isolation device and is absolutely still. This makes it necessary to stop the apparatus before the seismic vibration is transmitted to it.
An earthquake produces roughly three types of seismic waves: a P-wave, which propagates at a relatively high velocity and is readily dampened; an S-wave, which propagates at a velocity lower than that of the P-wave, is hardly dampened, and has the most significant influence on, for example, buildings, and surface waves (a Rayleigh wave and a Love wave), which have rarely posed problems up to now. At a position spaced apart from the hypocenter to a certain degree, surface waves often have only ultralow frequency components, which humans cannot feel. Since the resonance frequencies of surface waves are often enormously lower than those of various structures in this world, they have generally been thought to have no adverse influence on, for example, buildings. Therefore, it is a common practice to estimate the arrival of only P- and S-waves and to issue a warning against them (see Japanese Patent Laid-Open No. 2003-66152).
Various seismic vibration arrival estimation techniques have already been disclosed. Most of these techniques are associated with a method of empirically, approximately calculating the relationship between the time until seismic vibration arrives and the hypocentral distance. The relationship between P- and S-waves has already been announced to the public. According to “Real-time seismology”, University of Tokyo Press, issued Jan. 17, 2003, page 6, written by Masayuki Kikuchi, when an earthquake occurs in an upper portion of the earth's crust, a P-wave propagates at a velocity of 6 [km/s] and an S-wave propagates at a velocity of 3.5 [km/s].
However, a demand has arisen for an active vibration isolation device with a higher vibration isolation performance. To meet this demand, the natural frequency of the vibration isolation device itself is lowered, and its support object is controlled to be absolutely still with higher accuracy. This increases a risk that when seismic vibration occurs, a structure supporting a shaking vibration isolation device would come into contact with a structure, which is supported by the device and is absolutely still, due to the propagation of surface waves with ultralow frequencies, which have been thought to have no adverse influence on various structures up to now. That is, in addition to the P- and S-waves, surface waves with ultralow frequencies, which have been thought to have no adverse influence on various structures, are becoming non-negligible.
Surface waves are hardly dampened and circulate around the earth several times when a large-scale earthquake occurs. For this reason, seismic vibration sometimes continues for thirty minutes or more in each region around the world, including regions such as Great Britain in the United Kingdom, which has never been recorded to be a hypocenter.
As described above, surface waves may arrive even in a region which is sufficiently spaced apart from the hypocenter, and in which P- and S-waves are rarely observed. If a vibration isolation device suffers from an abnormality due to the propagation of such surface waves, which humans cannot feel, a lot of time is taken to investigate the cause of the abnormality. This prolongs the time taken to recover the vibration isolation device.